Chapter 14 Flashcards
What is a redox reaction?
A reaction where electrons are transferred
What is a loss of electrons called?
An oxidation reaction
What is a gain of electrons called?
Reduction
When an element is oxidised, what happens to the value of its oxidation number?
It increases
What can electrochemical cells be made from?
Two different metals dipped in salt solutions of their own ions and connected by a wire
What do electrochemical cells do?
They create electricity
Why is it a redox process within an electrochemical cell?
Because there’s always two reaction with an electrochemical cell, one being an oxidation and one being a reduction reaction.
What is the anode in an electrochemical cell?
The anode is the negative electrode
What is the cathode in an electrochemical cell?
The cathode is the positive electrode
Does reduction occur at the anode or at the cathode?
Reduction occurs at the anode
Does oxidation occur at the anode or at the cathode?
Oxidation occurs at the cathode
Why does the reactive metal become the anode and the less reactive metal becomes the cathode in an electrochemical cell?
Because the reactive metal gives up its electrons and is oxidised and becomes the anode, where electrons flow from
In the electrochemical cell with zinc and copper, where zinc gives up its electrons more easily than copper, describe the half equations:
Zn –> Zn2+ + 2e-
Cu2+ +2e- –> Cu
In an electrochemical cell, where do electrons flow from?
Through the wire from the most reactive to the least reactive metal
In an electrochemical cell, how can you measure the voltage between the two half cells?
The voltage between the two half cells
What does the voltage between two half cells show?
The cell potential, and the direction of electron flow
In an electrochemical cell, what are the half cells connected by?
A salt bridge
What are slat bridges made from?
Filter paper soaked in a salt solution, e.g. KNO3
In an electrochemical cell where neither reactants or products are solids, why use platinum as an electrode?
It can conduct electricity and is inert
What is the problem with using platinum as an electrode?
It is very expensive, so graphite is often used instead
When drawing electrochemical cells, is the anode drawn on the right or left?
On the right
Are the reactions in an electrode reversible?
Yes
When setting up an electrochemical cell, how do we clean the surfaces of the metals?
We use a piece of emery paper or sand paper
How do we clean any oil or grease from the electrodes?
We use some propanone
Once the electrodes have been cleaned, describe how you set up an electrochemical cell:
- Place each electrode into a beaker filled with a solution of ions of that metal
- Create a salt bridge linking them together
- Connect the electrodes to a voltmeter
What is an electrode potential?
A measure of how easy the substance in the half cell is oxidised
How does a potenital difference build up in the half cells?
As the substances in the half cells are oxidised or reduced, a potential difference is built up due to the difference in charge between the electrode and ions in solution
Does the half-equation with the more postive electrode potential go forwards or backwards?
Forwards
What are electrode potentials measured against?
Standard hydrogen potentials
What is the definition of the standard electrode potential?
The voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode
What are standard conditions?
- Conc must me 1 mol/dm
- 298K
- 100kPa
Is the standard hydrogen electrode shown on the left or on the right?
Always on the left
What does the hydrogen half cell have a voltage reading of?
0V
What is the equation for the reaction at the hydrogen electrode?
2H+ + 2e- –> H2
How do you calculate the cell potential?
E reduction - E oxidation
Why is the cell potential always a positive value?
The more negative E value is being subtracted from the more positive E value
Why are standard conditions used when measuring electrode potentials?
Because the equilibrium condition is affected by changing conditions, so standard conditions are used
In the conventional representation of electrochemical cells, which half cell goes on the left?
The half-cell with the more negative potential
In the conventional representation of electrochemical cells, where does the oxidised for go?
In the centre of the diagram
In the conventional representation of electrochemical cells, what do the double vertical lines represent?
The salt bridge
In the conventional representation of electrochemical cells, what do the single vertical lines represent?
The different states
In the conventional representation of electrochemical cells, what do commas represent?
Separate species that are in the same half-cell and in the same physical state
In the conventional representation of electrochemical cells, if either of the half-cells uses platinum, lead or other inert electrodes, how are they shown?
On the outside of the diagram, separated by a single vertical line
Wheat is the relationship between the size of the standard electrode potential and the reactivity of the metal?
The more reactive the metal, the more negative the value of the standard electrode potential
What is the relationship between the size of the standard electrode potential and the reactivity of a non-metal?
The more reactive a non-metal is, the more positive the value of the standard electrode potential is
How can you use electrode potentials to see if a reaction will happen or not?
If a reaction is thermodynamically feasible, the overall potential is positive
The overall cell potential is -1.4V, will the reaction occur?
No the reaction is not thermodynamically feasible
What happens during a disproportionation reaction?
An element is simultaneously oxidised and reduced
Describe 2 situations where the prediction for whether a reaction is feasible or not may cause the prediction to be wrong?
- When the conditions are not standard
- The reaction kinetics may not be favourable
What is cell potential proportional to?
Total entropy change and lnk
What are energy storage cells?
Batteries
What can fuel cells generate energy from?
Hydrogen and oxygen
What can hydrogen-oxygen fuel cells be used to power?
Electric vehicles
In hydrogen-oxygen fuel cells, what is the electrolyte?
An aqueous alkaline KOH solution
In hydrogne-oxygen fuel cells, what is fed into the anode and cathode?
The oxygen is fed into the positive cathode and the hydrogen is fed into the negative anode
In hydrogen-oxygen fuel cells, how do the electrons flow?
From the negative anode through an external circuit to the positive cathode
Where do the OH- ions pass through in a hydrogen-oxyegn fuel cell?
They pass through the anion-exchange membrane towards the negative electrode
What is the overall effect of hydrogen oxygen fuel cells?
H2 and O2 react to form water
At the anode, what does the platinum catalyst split the H2 into?
Protons and electrons
What does the polymer electrolyte membrane do?
It only allows the H+ across and forces the e- to travel around the circuit to get to the cathode
At the cathode, what does the O2 combine with?
The H+ from the anode and the e- from the circuit to form H2O
In a manganate ion, what is the oxidation number of manganese?
7+
What is a manganate ion?
Mn04 -
When a manganate ion is reduced, what does the oxidation number of manganese go from to?
+7 to 2+
What is a dichromate ion?
Cr207 2-
What is the oxidation number of chronium in dichromate?
+6
When dichromate is reduced, what does the oxidation number of chronium go from to?
+6 to +3
When performing a redox titration, why is excess dilute sulfuric acid added to the flask?
So that there are plenty of H+ ions to allow the oxidising agent to be reduced
When performing a redox titration with mangante ions, describe when you can tell when you reach the end point:
It is tainted with the purple colour of MnO4 -
What is an iodate ion?
I03 -
What does I03 - + 5I- + 6H+ react to form?
3I2 + 3H20
What do you react iodine with to calculate the number of moles of iodine produced?
Sodium thiosulfate: I2 + 2S2O3 2- –> 2I- + S4O6 2-
You add sodium thiosulfate one drop at a time until the blue colour dissapears, which indicates all the iodine has reacted
